Stabilization of benzene radical anion in ammonia clusters

We investigate electron attachment to large ammonia clusters doped with a single benzene (Bz) molecule (NH3)N·Bz, N̄ ≈ 320. Negatively charged clusters are probed by mass spectrometry, and the energy-dependent ion yields are derived from mass spectra measured at different electron energies. The ion efficiency curves of pure ammonia clusters exhibit two maxima. At around 6 eV, (NH3)n−1NH2− ions are produced via dissociative electron attachment (DEA) to NH3 molecules. (NH3)n− ions produced at this energy are formed by DEA followed by fragment caging. At low energies around 1.3 eV, only (NH3)n− ions are formed for cluster sizes n ≥ 35 that correspond to solvated electrons in ammonia clusters. The doped (NH3)n·Bz− cluster ions exhibit essentially the same energy dependence. The (NH3)n·Bz− ions are metastable and evaporate NH3 molecule(s), while pure (NH3)n− ions are stable. The lifetime for NH3 molecule evaporation from the Bz-doped clusters was estimated as τ ≈ 18 μs. We interpret the metastability of the doped clusters by the charge localization on a Bz− ion solvated in the ammonia, which is accompanied by an energy release leading to the evaporation of NH3 molecule(s).

shows a detail section of the mass spectra of negatively charged ammonia clusters doped with benzene molecules, which are shown in the main article in Fig. 6. The (NH3)nseries is labeled in the top spectrum measured at the electron energy of 6.0 eV and the same series is obvious in the bottom spectrum. The benzene doped Bz⋅(NH3)ncluster ions are labeled in the bottom spectrum (similar series is indeed observed in the top spectrum), however, the doped clusters are dominated by metastable decay, as explained in the main article.

Fig. S03
shows the positive ion mass spectra of ammonia (NH3)N clusters doped with benzene (Bz) molecules at different pickup pressures. The spectrum illustrates that under our current experimental conditions outlined in the main article corresponding to the top spectrum at the pickup pressure of 2.5×10 -5 mbar, the doped clusters contain prevailingly only a single NH3 dopant molecule. The abundances of Bz2(NH3)nH + clusters containing two Bz units are negligible compared to the ones with a single unit Bz2(NH3)nH + . Only at 4-times higher pickup pressure of 10×10 -5 mbar the Bz2(NH3)nH + series becomes clearly observable (bottom spectrum). It is worth noting that in the spectrum of negatively charged clusters (NH3)nBz -, the cluster ions with two Bz units could not be identified.

Isotope Contribution
In large clusters, isotope contributions can play an important role for mass peak assignments. In our present case the 15 N isotope (apart from the most abundant 14 N isotope) has to be considered with a natural abundance corresponding to a ≈ 0.004. The probability Pk n to find k isotopes 15 N in a cluster ion containing n ammonia molecules follows binomial distribution: For example, in (NH3)nH + cluster ion containing n = 40 ammonia molecules the probability of finding 15 N isotope corresponds to 14%. Therefore, next to the mass peak of this ion at m/z = 682 there is the isotope peak at 683 with abundance of about 14% of the main peak. A useful webtool can be implemented to calculate isotope contributions. 1

Metastable Decay
A reflectron TOF mass spectrometer can be used to determine a metastable decay of clusters. The methodology can be found, e.g., in earlier studies of metastable decay of ammonia and water clusters and references cited therein, 2,3 and also in basic literature about TOF mass spectrometers, e.g. Ref. 4. A brief explanation is given in the main article on page 4, where we argue that a mass peak assigned to (NH3)nclusters has actually three contributions: the stable (NH3)n − ions, the fragments from (NH3)n-1NH2 − ← (NH3)nNH2 − metastable decay, and the isotope contribution of the stable (NH3)n-1NH2 − ions. Similarly, the next smaller peak following the (NH3)n − ion contains the isotope contribution and also a contribution from the metastable decay of (NH3)n+1NH2 − ion where two NH3 molecules are successively evaporated. This is indicated for a particular example of n = 28 in Fig. S04 by the black dashed arrows. illustrates the geometrical arrangement and voltages in our reflectron TOF mass spectrometer (note that the voltages are negative as used for positive ion detection; for negative ions opposite polarities are used). Using these voltages and geometry, time of flight for different stable and metastable ions can be calculated. We have recently performed comprehensive experiments with positively and negatively charged water clusters, where their metastable behavior in our TOF was carefully analyzed and compared to previous works on water clusters. 2 This allows us to calibrate and confirm our calculations of flight times of different stable and metastable ions in our TOF, and compare them with our present TOF spectra for ammonia clusters.  For the particular example of the mass peaks around the ion (NH3)n -, n = 28, illustrated above we provide the calculated flight times in Tab. 1. From these flight times, it is obvious that the (NH3)27NH2cluster ions after the metastable evaporation of one NH3 molecule arrive at the detector with a delay ∆τ less than 13 ns after the stable (NH3)28ion; this corresponds to less than 1 mass unit in the mass scale. Thus, the stable (NH3)28ion and the metastable (NH3)27NH2ion after the evaporation of NH3 from (NH3)28NH2overlap in the mass spectrum as indicated in Fig.  S04. Similarly, the arrival time of the metastable ion after evaporation of two NH3 molecules (NH3)27NH2 -← (NH3)29NH2arrives close to hypothetical (NH3)28Hion.
Indeed, due to stray fields and inexact geometry, the calculated flight times are not exact and needed to be calibrated. However, the metastable ions can be proved by changing voltages in the reflectron on our TOF mass spectrometer. The method has been again described in details in the above mentioned publications. 2,3 Essentially, by changing the voltages, the metastable peaks shift in the mass scale, while the stable ions remain at their corresponding masses. This is illustrated below for an example of a doped (NH3)n+1Bzcluster ion.
The (NH3)nBzcluster ion that originates from (NH3)n+1Bz -, from which a monomer NH3 unit evaporated in the first field free region, arrives at the detector at a time close to a stable (NH3)nBzcluster ion. Under the focusing condition of our TOF, the displacement of this (NH3)nBzfragment ion almost exactly corresponds to ∆m/z = +1 from the stable (NH3)nBzion as illustrated above for the undoped clusters. Thus the (NH3)nBzfragment from the metastable (NH3)n+1Bzdecay overlaps with the isotope contribution of the stable (NH3)nBzions. However, the metastable peak can be distinguished from the isotope by changing the middle grid potential Ug of our reflectron.
In the mass calibrated scale, the metastable fragment peak position changes as a function of Ug, with respect to the stable (NH3)nBzion while the stable (NH3)nBzion position remains fixed. This is illustrated in Fig. S06 for the example of n = 41 cluster. The first smaller peak at m/z = 776.2 corresponds to the (NH3)nBz -, n = 41, cluster ion that did not evaporate any NH3 units in the TOF. The corresponding isotope contribution at m/z = 777.2 is indicated by a grey bar. The cyan bar corresponds to the isotope contribution of the metastable peak. Clearly, the actual mass peak is significantly higher and shifted in its position to a higher m/z = 777.6. More importantly, its position changes with Ug as indicated by small arrows at the top. Therefore, this peak cannot be assigned to the isotope contribution nor to a hypothetical Bz(NH3)nHion. It corresponds to the fragment ion of (NH3)nBz -, n = 42, from which one NH3 unit evaporated during its flight in the first field free region. This assignment is backed by the next smaller peak at m/z = 778.6. Its abundance roughly corresponds to the isotope of the major peak, i.e., the metastable fragment (NH3)nBz -, n = 41←42, as indicated by the higher gray bar, and its position follows the main peak with changing Ug. Thus this peak mainly corresponds to the isotope of the metastable (NH3)nBz -, n = 41←42 fragment ion.
It ought to be mentioned that we have recently done a very extensive investigation of metastable decay of positively and negatively charged large water clusters, 5 which enabled us to calibrate theoretically calculated flight times of cluster ions in our TOF mass spectrometer for various scenarios of metastable evaporation of monomer units. These calculations agree reasonably well with the metastable peak positions in the present TOF spectra (in the time domain) as already outlined above. More importantly the relative shift with Ug in the metastable peak position in time is well reproduced by the calculations. This further confirms our assignment of the metastable fragment ions.